Troops at one of the last major Ukrainian bases in Crimea defiantly sang their national anthem on Saturday as Russian forces stormed their compound and demanded their surrender.

Russian soldiers and armed pro-Kremlin toughs assaulted the base just after lunchtime, backed by armoured vehicles that smashed through the compound’s gates.

The raid came after the Ukrainians had pointedly ignored a deadline from the Russians to surrender, even holding a wedding celebration at the base for a newly-married serviceman and his bride.

As the Russian force, bristling with heavy machine guns, over-ran the base, the Ukrainians realised it was pointless to try to fight back. Instead, they lined up on the parade ground and burst into song as the intruders held them at gunpoint.

The symbolic gesture of defiance took place at the Belbek air base near Sevastopol, which was the scene of one of the first stand-offs in Crimea last month when the base’s commander had a face-to-face confrontation with Russian troops.

The extraordinary scenes unfolded as the West took further steps to isolate Moscow over its military annexation of Crimea, which saw rafts of EU and US sanctions slapped on President Vladimir Putin’s inner circle last week.

Poland urged the US to increase its military presence in Eastern Europe, a move that could reverse two decades of gradual demilitarisation of the old Cold War faultlines.

Meanwhile, William Hague, the Foreign Secretary, warned that the Kremlin was cultivating zones of influence in Europe, using its vast oil wealth to buy up huge stakes in the corrupt and ramshackle economies of the Balkans.

Diplomats have told The Telegraph that the Kremlin’s growing foothold in the region could upset plans for EU membership for both Bosnia and Serbia, both of which harbour hardline Serb factions that are strongly pro-Russian.

Saturday’s assault on the Belbek airbase took place around 4pm local time, when a large number of Russian Cossacks, backed by regular troops, arrived outside in armoured vehicles and jeeps mounted with heavy machine guns. Accompanying them were masked militiamen armed with Kalashnikovs, who threw up a cordon around the compound.

Shortly before the raid, Colonel Yuli Mamchur, the base’s Ukrainian commander, told reporters on the base that requests had been coming in nearly every day from the Russians telling them to leave.

“They are military, but it seems like they don’t understand something, as an officer I don’t have the right to leave my working place without an order,” he said.

Instead, the Ukrainians, who had had their weapons confiscated from them during a previous Russian incursion onto the base, attempted to carry on with life as normal. Film footage from the base released on Saturday showed a military wedding party underway, with soldiers cutting a cake, drinking Russian champagne, and toasting the newly weds – despite the Russian armoured vehicles looming nearer beyond the fence.

Having used an armoured vehicle to ram a truck that the Ukrainians had blocked the main gate gate with, Russian forces then overran the base. Explosions from stun grenades were heard and shots were fired, although only one Ukrainian soldier was believed to have been injured.

Inside, Colonel Yuli Mamchur, the commander of Ukrainian air force’s 204th tactical aviation brigade, drew up his troops on the parade ground, where they sang the Ukrainian national anthem.

As they did so, the Russian miliitiamen outside jeered and shouted “go home”. A man in camouflage and sunglasses dismantled a camera that had provided a live feed of the front gate to the base.

“We did not provoke this, this was brute force,” said a Ukrainian officer who identified himself only as Vladislav. “I do not know whether this base will be formally in Russian hands by the end of the day.”